This is the color of the split-bar widget when it is under the mouse
cursor but it is not being dragged.

The value may be either an RGB color object (see make-rgb), or a constant symbol or global
variable whose value is an RGB, or nil to use
the system-edge-shadow-color,
or else t to match the parent window's
background color, making the split-bar invisible. The value
may be set at creation time by passing the :color-under-mouse initarg to
make-instance, or any time
later by call (setf color-under-mouse).

The default value is t to avoid distracting
the user with a split-bar that flashes whenever the mouses happens to
move over it. The changing mouse cursor indicates that a drag can be
done there, even when the split-bar itself cannot be seen.